Ch. XXXV.] METAMORPHIC ROCKS. 591 



Origin of the Metamorphic Strata. 



Having said thus much of the mineral composition of the nietamorphic 

 rocks, I may combine what remains to be said of their structm-e and his- 

 tory with an account of the opinions entertained of their probable origin. 

 At the same time, it may be well to forewarn the reader that we are here 

 entering upon ground of controversy, and soon reach the limits where 

 positive induction ends, and beyond which we can only indulge in specu- 

 lations. It was once a favorite doctrine, and is still maintained by many, 

 that these rocks owe their crystalline texture, their want of all signs of 

 a mechanical origin, or of fossil contents, to a peculiar and nascent con- 

 dition of the planet at the period of their formation. The arguments in 

 refutation of this hypothesis will be more fully considered when I show, 

 in the last chapter of this volume, to how many different ages the 

 metamorphic formations are referable, and how gneiss, mica-schist, clay- 

 slate, and hypogene limestone (that of Carrara, for example) have been 

 formed, not only since the first introduction of organic beings into this 

 planet, but even long after many distinct races of plants and animals had 

 passed away in succession. 



The doctrine respecting the crystalline strata, implied in the name 

 metamorphic, may properly be treated of in this place ; and we must 

 first inquire whether these rocks are really entitled to be called stratified 

 in the strict sense of having been originally deposited as sediment from 

 water. The general adoption by geologists of the term stratified, as 

 applied to these rocks, sufficiently attests their division into beds very 

 analogous, at least in form, to ordinary fossiliferous strata. This resem- 

 blance is by no means confined to the existence in both occasionally of 

 a laminated structure, but extends to every kind of arrangement which 

 is compatible with the absence of fossils, and of sand, pebbles, ripple- 

 mark, and other characters which the metamorphic theory supposes 

 to have been obliterated by plutonic action. Thus, for example, we 

 behold alike in the crystalline and fossiliferous formations an alternation 

 of beds varying greatly in composition, color, and thickness. We 

 observe, for instance, gneiss alternating with layers of black hornblende- 

 schist, or of green chlorite-schist, or with granular quartz, or lime- 

 stone ; and the interchange of these different strata may be repeated 

 for an indefinite number of times. In the like manner, mica-schist 

 alternates with chlorite-schist, and with beds of pure quartz or of granu- 

 lar limestone. 



We have already seen that, near the immediate contact of granitic 

 veins and volcanic dikes, very extraordinary alterations in rocks have 

 taken place, more especially in the neighborhood of granite. It will be 

 useful here to add other illustrations, showing that a texture undis- 

 tinguishable from that which characterizes the more crystalline meta- 

 morphic formations has actually been superinduced in strata once fos- 

 siliferous. 



